Mail processing systems, such as, for example, mailing machines, and the like, often include different modules that automate the processes of producing mail pieces. The typical mailing machine includes a variety of different modules or sub-systems each of which performs a different task on the mail piece. The mail piece is conveyed downstream utilizing a transport mechanism, such as rollers or a belt, to each of the modules. Such modules could include, for example, a singulating module for separating a stack of mail pieces such that the mail pieces are conveyed one at a time along the transport path, a stripping/moistening module for stripping open the flap of an envelope, and wetting and sealing the glued flap of an envelope, a weighing module for weighing the mail piece, and a metering/printing module for storing postage amounts and applying evidence of postage either directly to the mail piece or to a tape to be applied to the mail piece. The mailing machine is controlled by a central processing unit that executes software stored in memory provided in the mailing machine. The exact configuration of the mailing machine is, of course, particular to the needs of the user.
Mailing machines for printing postage indicia on envelopes and other forms of mail pieces have enjoyed commercial success, both in the U.S. Postal Service and in industry mail rooms and private office environments. Generally speaking, there are many different types of mailing machines, ranging from relatively small units, which handle only one mail piece at a time to large, multi-functional units, which can separate, feed, weigh, print postage indicia on and stack thousands of mail pieces per hour in a continuous stream operation. Thus, the modern mailing machine plays an important role in facilitating the rapid and efficient movement and other handling of mail.
Mailing machines have traditionally been capable of printing postage indicia either directly on mail pieces, or on pieces of tape, which are then attached to mail pieces. Typically, the mailing machine is set to print the postage indicia on envelopes as they are fed seriatim along a feed deck by a suitable feeding mechanism, the printing operation being carried out by a printing device that is part of a postage meter component of the mailing machine. The postage meter component has various control devices by which it can be set to print a predetermined amount of postage, together with other settable information, such as a date, within a pre-set design, the selectable information and the pre-set design all constituting the aforementioned postage indicia.
Many mailing machines currently utilize digital printing technology to print images in the form of indicia that evidences payment of postage on mail pieces. Digital printing technology includes thermal ink jet (bubble jet), piezoelectric ink jet, thermal printing techniques, and LED and laser xerographic printing that all operate to produce images by dot-matrix printing. In dot-matrix ink jet printing individual print elements in the print head (such as resistors or piezo electric elements) are either electronically stimulated or not stimulated to expel or not expel respectively, drops of ink from a reservoir onto a substrate. Thus, by controlling the timing of the energizing of each of the individual print elements in conjunction with the relative movement between the print head and the mail piece, a dot-matrix pattern is produced in the visual form of the desired indicia.
Regardless of which type of printing device is utilized in the postage meter, as noted above it is sometimes necessary to print the postage indicia on a strip of tape, either gummed or adhesive backed, because it is not possible to feed the mail piece on which it is desired to apply the postage indicia through the mailing machine. In many situations, for example, the mail piece may be too thick or too large in area, such as, for example, a parcel or package, to be fed through the normal feeding path of the mailing machine, or it may contain delicate material that could be damaged by the pressure exerted by the transport device of the mailing machine. Thus, there are numerous occasions in the normal operating situations of a mailing machine where the postage indicia simply cannot be applied directly to the mail piece and must be applied to a strip of tape, which is then suitably adhered to the mail piece.
There are problems, however, with the use of conventional digital printing technology, especially in mailing systems with very high throughputs. The ink ejection process, as described above, is physically limited by inherent frequency constraints. For example, the timing of the energizing of each of the individual print elements is limited within a maximum firing cycle for the print element. If the print medium is moving very fast, as in high throughput mailing systems, it may not possible to print an image with sufficient resolution that will not be distorted because the print elements of the print head cannot be energized quickly enough to keep pace with the movement of the print medium. To overcome this problem, high throughput mailing systems have been provided with multiple print heads, separated by some distance, typically several inches, which print in cooperation with each other to form a final image. In such systems, for example, the first print head will print all of the even columns of the image on the print medium, and the second print head will print all of the odd columns of the image on the print medium. Accordingly, the print process could be thought of as printing two separate copies of the image at half the horizontal resolution, and combining the images into a single image with the full horizontal resolution. Thus, by passing the print medium past both print heads, a complete image will be printed. By accurately controlling the firing of the second print head with respect to the first print head, a complete image can be printed that has sufficient resolution and is not distorted, regardless of the speed of the print medium.
While the use of multiple print heads solves the printing issue with respect to print medium that is moving very fast, it also introduces new problems. For example, since the printing process is essentially performed by printing two separate copies of the image at half the resolution, care must be taken to ensure that the two separate images cannot be printed on different media. This is especially true if the mailing machine is operating in the tape mode, i.e., where the indicia is printed on a strip of tape, as the tape is a flexible media that can be easily manipulated in the physical space between the print heads. If the same indicium can be printed on different pieces of tape (albeit in lower resolution), it is possible to defraud the postal authority of postage funds, since the indicium will have been accounted for only once, and can then be used to evidence payment of postage on two different mail pieces. This type of fraud is commonly referred to as “2 for 1” fraud.
Thus, there exists a need for improving the security of the printing process of postage evidencing on tape media in mail processing systems that utilize multiple print heads.